Mixer of communication system

ABSTRACT

A mixer of a communication system. The communication system has an antenna, low noise amplifier, a mixer, a local oscillator and an intermediate frequency filter. The mixer has a mixer circuit, a gain amplified circuit, a voltage auto-tracking circuit and a direct current voltage generating circuit. The direct current voltage generating circuit can reduce the output power of the local oscillator, extend the lifetime of the battery used in the mobile communication system, and reduce the distortion of harmonic wave of the local oscillator to reduce the noise figure of the mixer. Using the voltage auto tracking circuit and the gain amplified circuit, the linearity of the mixer is enhanced, and the conversion gain of the mixer is adjusted by varying the load resistors.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 89120915, filed Oct. 6, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a mixer of a communication system.More particularly, this invention relates to a mixer with the functionsof increasing conversion gain, reducing noise figure and enhancinglinearity.

2. Description of the Related Art

FIG. 1 is a block diagram showing a receiving part of a conventionalcommunication system. A radio frequency signal is received by an antenna10 and sent to a low noise amplifier 12. Being amplified by the lownoise amplifier 12, a radio frequency P_(i) is output to a mixer 14. Ifthe radio frequency signal P_(i) has a frequency of 1800 kHz, a highfrequency signal of 1810 kHz is fed into the mixer 14 from a localoscillator 16.

After receiving the radio frequency P_(i) and the high frequency signal,the mixer 14 outputs an output signal P₀ to an intermediate frequencyfilter 18. The high frequency signal is filtered by the intermediatefrequency filter 18, and an intermediate frequency signal is output to asub-ordinary circuit (not shown). The frequency of the intermediatefrequency signal is about 10 kHz.

FIG. 2 shows a circuit diagram of a Gilbert multiplier. In FIG. 2, aGilbert multiplier 20 is used as a mixer. Three differential amplifiersare assembled by an NMOS M1 22 and an NMOS M2 24, an NMOS M3 26 and anNMOS M4 28, and an NMOS M7 30 and an NMOS 32, respectively. The NMOS M734 is used as a current source.

In FIG. 2, the Gilbert multiplier used as a mixer circuit 20 has anoutput power signal P₀ as:

P₀=P_(LO)×P_(i)

P_(LO)=A₂ sin ω_(LO)t

P_(i)=A₁ sin ω_(i)t $\begin{matrix}{P_{0} = {A_{1}A_{2}{\sin \omega}_{i}{t \cdot {\sin \omega}_{LO}}t}} \\{= {{1/2}\quad A_{1}{A_{2}\left\lbrack {{{\cos \left( {\omega_{LO} - \omega_{i}} \right)}t} - {{\cos \left( {\omega_{LO} - \omega_{i}} \right)}t}} \right.}}}\end{matrix}$

wherein, P_(i) is the input power signal input from an RF input terminalto the mixer circuit 20, and P_(LO) is the input power signal input fromthe LO input terminal to the mixer circuit 20. A₁ is the amplitude ofthe input power signal P_(i), and A₂ is the amplitude of the input powersignal P_(LO). ω_(l) is the frequency of the input power signal P_(i),and ω_(LO) is the frequency of the input power signal P_(LO).ω_(i)+ω_(LO) is a high frequency signal, and ω_(i)−ω_(LO) anintermediate frequency signal. Through the intermediate frequencyfilter, the intermediate frequency signal ω_(i)−ω_(LO) can be obtainedfrom the high frequency signal ω_(i)+ω_(LO).

The characteristics of the mixer can be described by the followingparameters:

(1) The conversion gain C. G.=P₀/P_(I);

(2) The noise figure NF=(S₀/N₀)/(S_(i)/N_(i)); and

(3) The linearity as shown as the curve of the mixer in FIG. 3. When anactual curve is different from the ideal curve with 1 dB, therelationship between the input power and output power is judged todetermine the degree of the linearity of the mixer.

In a mobile communication system, a large conversion gain and a smallnoise figure of the mixer can be obtain by increase the output power ofthe local oscillator input to the mixer. However, to increase the outputpower of the local oscillator increase the power consumption of themobile communication system to reduce the lifetime of the battery of themobile communication system. If the output power of the local oscillatoris decreased, the linearity of the mixer is decreased.

SUMMARY OF THE INVENTION

The invention provides a mixer of a communication system to reduce theoutput power of the local oscillator, extend the lifetime or operationtime of the battery, suppress the distortion of harmonic wave of thelocal oscillator, and to reduce the noise figure and enhance thelinearity of the mixer.

A mixer of a communication system comprising an antenna, a low noiseamplifier, a mixer, a local oscillator and an intermediate frequencyfilter is provided in the invention. The mixer comprises a mixer circuitto perform a multiplication on a radio frequency received from theantenna and a radio frequency generated from the local oscillator. Again amplified circuit is coupled a power supply terminal of the mixercircuit to increase the linearity of the mixer circuit. An voltageauto-tracking circuit is coupled to an output terminal of the gainamplified circuit to control the direct voltage output from outputterminal of the gain amplified circuit. A direct current voltagegenerating circuit is coupled to the input terminal of the localoscillator of the mixer circuit to generate a fix threshold voltage.

The voltage auto-tracking circuit further comprises a resistor, acapacitor, an operation amplifier and an NMOS. The resistor has oneterminal coupled to the output terminal of the gain amplified circuit.One terminal of the capacitor is coupled to ground, while the otherterminal of the capacitor is coupled to the other terminal of theresistor. The positive terminal of the operation amplifier is coupled tothe other terminal of resistor, and the negative terminal of theoperation amplifier is coupled to a reference voltage. The NMOS has adrain coupled to a power source, a source coupled to output terminal ofthe gain amplified circuit, and a gate coupled to an output terminal ofthe operation amplifier. A base of the NMOS is coupled to the source ofthe NMOS. The NMOS is used to provide a direct current.

The gain amplified circuit comprises a first current mirror circuit, asecond current mirror circuit and a third current mirror circuit. Thefirst mirror circuit has an impedance with one terminal coupled to apower source and the other end coupled to a first power input terminalof the mixer circuit. The first mirror circuit comprises a direct sourcecoupled to the power source. The first current mirror circuit provides adirect current to the mixer circuit. The second current mirror circuitcomprises an impedance with one terminal coupled to the power source andthe other terminal coupled to a second power input terminal of the mixercircuit. The second current mirror circuit provides a direct current tothe mixer circuit. The third mirror circuit comprises an impedance withone terminal coupled to a current source of the first current mirrorcircuit and the other terminal coupled to ground. The third currentmirror circuit comprises a current source with one terminal coupled toan output terminal of the gain amplified circuit, and the other endcoupled to ground. The third current mirror circuit also provides adirect current.

In the direct current voltage generating circuit, a first PMOS has asource coupled to the power source, and a base coupled to its the drain.A first NMOS has a drain coupled to the drain of the first PMOS, a gatecoupled to its own drain, and a base of the first NMOS coupled to itsown source. A second NMOS has a drain coupled to the source of the firstNMOS, a gate coupled to its own drain, and a base coupled its ownsource. A second PMOS has a source coupled to the power source, a gatecoupled to the gate of the first PMOS, a drain coupled to its own gate,and a base coupled to its drain. A third NMOS has a drain coupled to thedrain of the second PMOS, a gate coupled to the gate of the first NMOS,and a base coupled to its source. A fourth NMOS has a drain coupled tothe source of the third NMOS, a gate coupled to the gate of the secondPMOS, a source coupled to the source of the second NMOS, and a basecoupled to its own source. A fifth NMOS has a gate coupled to a biaspower source, a source coupled to ground, and a base coupled to its ownsource. A third PMOS has a source coupled to the power source, a gatecoupled to the drain of the second PMOS, a drain coupled to the outputterminal of the direct current voltage generating circuit, and a basecoupled to its own drain. A sixth NMOS has a drain coupled to the drainof the third PMOS, a gate coupled to its own drain, a source coupled tothe source of the third NMOS, and a base coupled to its own source.

Therefore, the invention provides a mixer of a communication systemusing direct current voltage generating circuit. The output power of thelocal oscillator is reduced, the lifetime of the battery used in thecommunication system is extended, and the harmonic wave distortion ofthe local oscillator is reduced to reduce the noise figure of the mixer.Using the voltage auto-tracking circuit and the gain amplified circuit,the linearity of the mixer is enhanced, and the conversion gain of themixer can be adjusted by varying the load resistor.

Both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional communication system;

FIG. 2 is circuit diagram of a Gilbert multiplier;

FIG. 3 is a graph showing a linearity of a mixer;

FIG. 4 is a block diagram showing the mixer provided in the invention;

FIG. 5 shows a relationship between a conversion gain and the noisefigure, and s power output of a local oscillator;

FIG. 6 shows a relationship between the transistor and the power spectradensity the generated noise;

FIG. 7 shows the characteristics of the NMOS;

FIG. 8 shows a relationship between the local output power and theconversion gain;

FIG. 9 shows an equivalent circuit of a direct current voltagegenerating circuit provided in the invention;

FIG. 10 shows waveforms of voltage and current of a transistor used as aswitch in the mixer circuit;

FIG. 11 shows the relationship between an output current and an inputvoltage of the differential pair transistors; and

FIG. 12 shows a circuit diagram of the gain amplified circuit and thevoltage auto-tracking circuit of the mixer provided in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows a block diagram of a mixer 40. The mixer 40 comprises amixer circuit 42, a voltage auto-tracking circuit 44, a gain amplifiedcircuit 46 and a direct current voltage generating circuit 48. TheGilbert multiplier as shown in FIG. 2 can be adapted as the mixercircuit 42. The mixer circuit 42 performs a multiplication on a receivedradio frequency signal and a radio frequency generated from a localoscillator (referring to FIG. 1) to obtain a high frequency signal andan intermediate frequency signal. (The high frequency is filtered out inthe intermediate frequency filter in the sub-ordinary circuit.).

The voltage auto-tracking circuit 44 is coupled to an output terminalP_(o) of the mixer circuit 42 to control the output direct currentvoltage from the mixer circuit 42, so as to obtain a maximum amplitudeof the output signal of the mixer circuit 42. The gain amplified circuit48 is coupled to a power supply terminal of the mixer circuit 42. Thegain amplified circuit 48 increases the output gain of the mixer circuit42 and enhance the linearity of the mixer circuit 42. The direct currentvoltage generating circuit 46 is coupled to an input terminal of thelocal oscillator of the mixer circuit 42 to generate a fixed thresholdvoltage. The power consumption of the local oscillator (referring toFIG. 1) can thus be reduced, and improved conversion gain and noisefigure can be obtained in the mixer 40.

FIG. 5 shows a curve of a relationship between the conversion gain andnoise figure and the power of the local oscillator. The conversion gainand figure noise while using the Gilbert multiplier as the mixer isshown as FIG. 5. In the section I of FIG. 5, the signal output isproportion to the power of the local oscillator when the power is small.The mixer is like an ideal multiplier in this second. In section II, allthe transistors used as switches (the NMOS M1 to M4 as shown in FIG. 2)are conducted. In this section, the minimum noise figure and the largestconversion gain of the mixer can be obtained. In section III, too muchpower of the local oscillator causes the decrease of the conversiongain.

In a mobile communication system, to reduce the power of the localoscillator can increase the output power of the oscillator, so as toextend the lifetime of the battery used in the mobile communicationsystem, reduce the harmonic wave distortion. Furthermore, the noisefigure of the mixer can be increased to increase the isolation betweenlocal oscillator-radio frequency (LO-RF) and the localoscillator-Intermediate frequency (LO-IF).

FIG. 6 shows a relationship between the transistor and the power spectradensity (PSD) generated noise of the generated noise. In FIG. 6, whenthe voltage V_(LO) is higher than ±V_(x), the switch transistor isconducted and enters a triode region. Meanwhile, the current can flowthrough the switch transistor, and the power spectra density of thechannel resist noise of the switch transistor entering the outputterminal of the mixer can be expressed as:$S_{n12}^{0} = {16{{KT\gamma}\left( \frac{{gm1} \cdot {gm2}}{{gm1} + {gm2}} \right)}}$

wherein gm is the conductance, K is the Boltzman constant, T is theabsolute temperature, and γ=⅔ for a long channel transistor.

In FIG. 6, when V_(LO) is higher than ±V_(x), the noise does not enterthe output terminal of the mixer. To improve the performance of themixer, the duration Δ for the magnitude of V_(LO) between ±V_(x) has tobe decreased. To increase the power of the local oscillator or reducethe bias current can also achieve the objective. However, to increasethe power of the local oscillator is disadvantageous to the mobilecommunication system, and to increase the bias current results in aworse linearity and a decreased gain. Another method is to reduce thedrain voltage of a switch transistor and to have the bias fall at theedge between the saturation region and the triode, the switch transistorcan thus easily reaches the triode region.

FIG. 7 shows the characteristics of the NMOS device. In FIG. 7, threeoperation points A, B and C are selected. Point A is in the saturationregion, point B falls at the edge between the saturation region and thetriode region, and point B is in the triode region. In FIG. 8, the curveA represents the relationship when the switch transistor operates at abias at the point A in FIG. 7. The curve B represents the relationshipwhen the switch transistor operates at a bias at the point B in FIG. 7.The curve C represents the relationship when the switch transistoroperates at a bias at the point C in FIG. 7. Apparently, the biasoperation at point A requires a large local oscillator power, the biasoperation at point C does not only require a large local oscillatorpower, but also reduce the conversion gain. It is because that when theswitch transistor requires a high voltage to leave the triode region. Inaddition, the leakage of the switch transistor also causes theconversion gain decrease. To have the smallest local oscillator power,the bias of the switch transistor has to be set at the edge between thesaturation region and the triode region, that is, at point B.

The power of the battery decays as the mobile communication system beingused, therefore, the voltage of the battery gradually decreases to causea difference in voltage at the drain of the switch transistor. Thus, thebias operation point of switch transistor varies to affect theconversion gain and noise figure of the mixer. A direct current voltagegenerating circuit is thus designed to enable the bias operating pointof the transistor varies as the voltage of the battery changes.

FIG. 9 shows a circuit diagram of the direct current voltage generatingcircuit 110. In FIG. 9, a PMOS P1 90 has a source coupled to a powersource Vdd, and a base coupled to its own drain. An NMOS N1 92 has adrain coupled to the drain of the PMOS P1 90, a gate coupled to its owndrain, a base coupled to its own source. An NMOS N2 94 has a draincoupled to the source of the NMOS N1 92, a gate coupled to its owndrain, a base coupled to its own source.

A PMOS P2 96 has a source coupled to the power source Vdd, a gatecoupled to the gate of the PMOS P1 90, a drain coupled to its own gate,and a base coupled to its own drain. An NMOS N3 98 as drain coupled tothe drain of the PMOS P2 96, a gate coupled to the gate of the PMOS P296, a and base coupled to its own source. An NMOS N4 100 has a draincoupled to the source of the NMOS N3 98, a gate coupled to the gate ofthe PMOS P2 96, a source coupled to the source of the NMOS N2 94, and abase coupled to its own source. An NMOS N5 102 has a drain coupled tothe source of the NMOS N4 100, a gate coupled to a bias power sourceBias, a source coupled to ground, and a base coupled to its own source.

A PMOS P3 104 has a source coupled to the power source Vdd, a gatecoupled to the drain of the PMOS P2 96, a drain coupled to an outputterminal of the direct current voltage generating circuit 110, and abase coupled to its own drain. An NMOS N6 104 has a drain coupled to thedrain of the PMOS P3 104, a gate coupled to its own drain, a sourcecoupled to the source of the NMOS N3 98, and a base coupled to its ownsource.

In FIG. 9, the power source Vdd providing the power to the directcurrent voltage generating circuit 110 is the same as the power sourceproviding the power to the mixer circuit (referring to FIG. 2). When thepower source Vdd is dropped, the drain voltage of the switch transistorof the mixer circuit is consequently dropped (referring to FIG. 2). Theoutput voltage from the direct current voltage generating circuit 110 tothe input terminal of the local oscillator is varied with the drainvoltage of the switch transistor to ensure that the switch transistor isoperated at the edge between the saturation region and the trioderegion.

In semiconductor fabrication process, the threshold voltage V_(t) ofevery transistor is not identical. The drain region is affected by thethreshold voltage V_(t) of the switch transistor. The direct currentgenerating circuit 110 can also enable the switch transistor to beoperated at an edge between the saturation region and the triode regionto obtain the optimum conversion gain and noise figure.

In FIG. 2, the switch transistors (that is, NMOS M1 22 to NMOS M4 28)has a bias V_(sd)=V_(gs)−V_(t). If the output voltage of the mixercircuit 20 has a large amplitude, the switch transistors are reverselyconducted (that is, the polarities of the drain and source areinterchanged). This phenomenon is shown as the waveforms of the voltageand current of the switch transistors of the mixer circuit in FIG. 10.In FIG. 10, the larger output voltage of the mixer circuit 20 results inthe current flowing through the NMOS M1 22 smaller than zero. Thecurrent leaked from the NMOS M1 22 causes the output power of the mixercircuit 22 dropped to reduce the conversion gain and to deteriorate thelinearity.

Another reason for the deterioration of linearity is the saturationcurrent flowing through the differential pair transistors NMOS M5 30 andNMOS M6 32 of the mixer circuit 20 as shown as the relationship betweenthe output current and input voltage of the differential pairtransistors in FIG. 11. When the input voltage of the differential pairtransistors NMOS M5 30 and NMOS M6 32 of the mixer circuit 20 is higherthan 400 mV, the direct bias current flows through only one of thetransistors to cause the output current of the mixer circuit 20saturated.

FIG. 12 shows a circuit diagram of the gain amplified circuit and thevoltage auto-tracking circuit in the mixer. The gain amplified circuit120 comprises one current mirror circuit 126, a second current mirrorcircuit 128 and a third current mirror circuit 130. The impedance 1/gm132 of the first current mirror circuit 126 has a first terminal coupledto the power source Vdd and the other terminal coupled to the powerinput terminal of the mixer circuit 124. The first current mirrorcircuit 126 further comprises a current source 134 with one terminalcoupled to the power source Vdd. The first current mirror circuit 126provides a direct current to the mixer circuit 124.

The impedance 1/gm 136 of the second current mirror circuit 128 has oneterminal coupled to the power source Vdd and the other terminal coupledto another power input terminal of the mixer circuit 124. The secondcurrent mirror circuit 128 further comprises a current source 138 withone terminal coupled to the power source Vdd and the other terminalcoupled to an output terminal of the gain amplified circuit 120. Thesecond current mirror circuit 128 provides a direct current to the mixercircuit 124.

The impedance 1/gm 140 of the third current mirror circuit 130 has oneterminal coupled to the other terminal of the current source 134 of thefirst current mirror circuit 126 and the other terminal coupled toground. The current source 142 of the third current mirror circuit 130has one terminal coupled to the output terminal of the gain amplifiedcircuit 120 and the other terminal coupled to ground. The third currentmirror circuit generates a direct current source.

As mentioned above, too large voltage amplitude of the mixer circuit 124causes the deterioration of linearity. Therefore, the current mirrortype with low impedance gain amplified circuit 120 improves thesituation. In FIG. 12, in the gain amplified circuit 120, apart from thehigh input impedance point A, the rests are all low input impedancepoints. The low input impedance reduces the RC time constant of themixer (referring to FIG. 1) and raises the frequency response to allowan operation with a higher output frequency.

In FIG. 12, the voltage auto-tracking circuit 122 comprises a resistor R144, a capacitor C 146, an operation amplifier 148 and an NMOS 150. Theresistor R 144 has one terminal coupled to the output terminal of thegain amplified circuit 120, that is, point A. The capacitor C 146 hasone terminal coupled to the other terminal of the resistor R 144, andthe other terminal coupled to ground. The operation amplifier 148 has apositive terminal coupled to the other terminal of the resistor R 144and a negative terminal coupled to a reference voltage ½ Vdd. Theoperation amplifier 148 controls the direct current voltage at theoutput terminal of the gain amplified circuit 120 at ½ Vdd. The NMOS M150 has a drain coupled to the power source Vdd, a source coupled to theoutput terminal of the gain amplified circuit 120, a gate coupled to theoutput terminal of the operation amplifier 148, and a base coupled toits own source.

In FIG. 12, the NMOS M 150 provides a direct current to the outputterminal of the gain amplified circuit (that is, point A) to change theoutput voltage of the gain amplified circuit 120. The NMOS M 150 iscontrolled by the operation amplifier 148. Being fed back, the voltageat the output terminal of the gain amplified circuit 120 is controlledat ½ Vdd. To control the output voltage of the gain amplified circuit120 at ½ Vdd provides a maximum amplitude without distortion, so thatthe linearity is enhanced. Moreover, the output of the gain amplifiedcircuit 120 is a single terminal signal current 2KI. The load resistor(not shown) determines the magnitude of the output power of the mixer.Therefore, the conversion gain can be adjusted according to thevariation of the load resistor.

Thus, the invention provides a mixer of a communication system usingdirect current voltage generating circuit to reduce the output power ofthe local oscillator, extend the battery lifetime of a mobilecommunication system and reduce the harmonic wave distortion to reducethe noise figure of the mixer. Using the voltage auto-tracking circuitand the gain amplified circuit, the linearity of the mixer is enhanced.In addition, by adjusting the load resistor, the conversion gain can beimproved.

Other embodiments of the invention will appear to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples to be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A mixer of a communication system, thecommunicating system comprising an antenna, a low noise amplifier, themixer, a local oscillator and an intermediate frequency filter, themixer comprising: a mixer circuit, to perform a multiplication on aradio frequency received from the antenna and a radio frequency signalgenerated by the local oscillator; a gain amplified circuit, coupled toa power supply terminal of the mixer circuit to increase a linearity ofthe mixer circuit; a voltage auto-tracking circuit, coupled to an outputterminal of the gain amplified circuit to control a direct currentoutput from an output terminal of the gain amplified circuit; and adirect current voltage generating circuit, coupled to an input terminalof a local oscillator of the mixer circuit to generate a fixed thresholdvoltage.
 2. The mixer according to claim 1, wherein the voltageauto-tracking circuit further comprises: a resistor, with one terminalcoupled to the output terminal of the gain amplified circuit; acapacitor, with one terminal coupled to the other terminal of theresistor and the other terminal coupled to ground; an operationamplifier, with a positive terminal coupled to the other terminal of theresistor, a negative terminal coupled to a reference voltage to controlthe direct current output from the output terminal of the gain amplifiedcircuit; and an NMOS, with a drain coupled to a power source, a sourcecoupled to the output terminal of the gain amplified circuit, a gatecoupled to the output terminal of the operation amplifier, and a basecoupled to the source of the NMOS that provides a direct current.
 3. Themixer according to claim 1, wherein the gain amplified circuit furthercomprises: a first current mirror circuit, further comprising: animpedance with one terminal coupled to the power source and the otherterminal coupled to a first power input terminal of the mixer circuit;and a current source with one terminal coupled to the power source togenerate a direct current to the mixer circuit; a second current mirrorcircuit, further comprising: an impedance with one terminal coupled tothe power source and the other terminal coupled to a second power inputterminal of the mixer circuit; and a current source with one terminalcoupled to the power source and the other terminal coupled to outputterminal of the gain amplified circuit to generate a direct current tothe mixer circuit; and a third current mirror circuit, furthercomprising: an impedance, with one terminal coupled to the otherterminal of the current source of the first current mirror circuit andthe other terminal coupled to ground; and a current source with oneterminal coupled to the output terminal of the gain amplified circuitand the other terminal coupled to ground, the third current mirrorcircuit generating a direct current.
 4. The mixer according to claim 1,wherein the direct current voltage generating circuit further comprises:a first PMOS, having a source coupled to the power source, and a baseand a drain coupled to each other; a first NMOS, having a drain coupledto the drain of the first PMOS, a gate and coupled to the drain of thefirst NMOS, and a base and a source coupled to each other; a secondNMOS, having a drain coupled to the source of the first NMOS, a gatecoupled to the drain of the second NMOS, and a base and a source coupledto each other; a second PMOS, having a source coupled to the powersource, a gate coupled to the gate of the first PMOS, a drain coupled tothe gate of the second PMOS, and a base coupled to the drain of thesecond PMOS; a third NMOS, having a drain coupled to the drain of thesecond PMOS, a gate coupled to the gate of the first NMOS, a base and asource coupled to each other; a fourth NMOS, having a drain coupled tothe source of the third NMOS, a gate coupled to the gate of the secondPMOS, a source coupled to the source of the second NMOS, and a basecoupled to the source to the fourth NMOS; a fifth NMOS, having a draincoupled to the source of the fourth NMOS, a gate coupled to a bias powersource, a source couple to ground and a base coupled to the source ofthe fifth NMOS; a third PMOS, having a source coupled to the powersource, a gate coupled to the drain of the second PMOS, a drain coupledto the output terminal of the direct current voltage generating circuit,and a base coupled to the drain of the third PMOS; and a sixth NMOS,having a drain coupled to the drain of the third NMOS, a gate coupled tothe drain of the sixth NMOS, a source coupled to the source of the thirdNMOS and a base coupled to the source of the sixth NMOS.